Split driver backlight systems and methods

ABSTRACT

Aspects of the subject technology relate to control circuitry for light-emitting diodes. The control circuitry may operate a light-emitting diode using a multi-peak pulse-width-modulation signal. The control circuitry may include a multi-stage driver having a relatively larger driver stage for providing a direct current through a light-emitting diode and a relatively smaller driver stage configured to cooperate with a pulse-width-modulation controller to pulse-width-modulate a current through the light-emitting diode.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/546,456, entitled “SPLIT DRIVER BACKLIGHTSYSTEMS AND METHOD,” filed on Aug. 16, 2017, which is herebyincorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

The present description relates generally to electronic devices withdisplays, and more particularly, but not exclusively, to electronicdevices with displays having backlights.

BACKGROUND

Electronic devices such as computers, media players, cellulartelephones, set-top boxes, and other electronic equipment are oftenprovided with displays for displaying visual information. Displays suchas organic light-emitting diode (OLED) displays and liquid crystaldisplays (LCDs) typically include an array of display pixels arranged inpixel rows and pixel columns. Liquid crystal displays commonly include abacklight unit and a liquid crystal display unit with individuallycontrollable liquid crystal display pixels.

The backlight unit commonly includes one or more light-emitting diodes(LEDs) that generate light that exits the backlight toward the liquidcrystal display unit. The liquid crystal display pixels are individuallyoperable to control passage of light from the backlight unit throughthat pixel to display content such as text, images, video, or othercontent on the display.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain features of the subject technology are set forth in the appendedclaims. However, for purpose of explanation, several embodiments of thesubject technology are set forth in the following figures.

FIG. 1 illustrates a perspective view of an example electronic devicehaving a display in accordance with various aspects of the subjecttechnology.

FIG. 2 illustrates a block diagram of a side view of an electronicdevice display having a backlight unit in accordance with variousaspects of the subject technology.

FIG. 3 illustrates a schematic diagram of light-emitting diode (LED)driver circuitry for direct and pulse-width-modulation (PWM) currentcontrol in accordance with various aspects of the subject technology.

FIG. 4 illustrates a brightness control scheme for direct and PWMlight-emitting diode (LED) currents in accordance with various aspectsof the subject technology.

FIG. 5 illustrates a brightness control scheme for multi-peak PWMlight-emitting diode (LED) currents in accordance with various aspectsof the subject technology.

FIG. 6 illustrates LED headroom voltages generated using direct and PWMlight-emitting diode (LED) currents in accordance with various aspectsof the subject technology.

FIG. 7 illustrates LED headroom voltages generated using direct andmulti-peak PWM light-emitting diode (LED) currents in accordance withvarious aspects of the subject technology.

FIG. 8 illustrates a schematic diagram of light-emitting diode (LED)driver circuitry having multiple drivers each for providing direct andpulse-width-modulation (PWM) current control in accordance with variousaspects of the subject technology.

FIG. 9 illustrates a schematic diagram of light-emitting diode (LED)driver circuitry having a first driver for providing direct LED currentcontrol and a second driver for providing pulse-width-modulation (PWM)LED current control in accordance with various aspects of the subjecttechnology.

FIG. 10 illustrates a schematic diagram of light-emitting diode (LED)driver circuitry having current scaling circuitry for each of a firstdriver for providing direct LED current control and a second driver forproviding pulse-width-modulation (PWM) LED current control in accordancewith various aspects of the subject technology.

FIG. 11 illustrates a brightness control scheme for direct and PWMlight-emitting diode (LED) currents with a trimmalbe and scalable kneepoint in accordance with various aspects of the subject technology.

FIG. 12 is a flow chart of illustrative operations that may be performedfor multi-peak PWM LED current control in accordance with variousaspects of the subject technology.

FIG. 13 is a flow chart of illustrative operations that may be performedfor dual driver LED current control in accordance with various aspectsof the subject technology.

DETAILED DESCRIPTION

The detailed description set forth below is intended as a description ofvarious configurations of the subject technology and is not intended torepresent the only configurations in which the subject technology may bepracticed. The appended drawings are incorporated herein and constitutea part of the detailed description. The detailed description includesspecific details for the purpose of providing a thorough understandingof the subject technology. However, it will be clear and apparent tothose skilled in the art that the subject technology is not limited tothe specific details set forth herein and may be practiced without thesespecific details. In some instances, well-known structures andcomponents are shown in block diagram form in order to avoid obscuringthe concepts of the subject technology.

The subject disclosure provides electronic devices such as cellulartelephones, media players, tablet computers, laptop computers, set-topboxes, smart watches, wireless access points, and other electronicequipment that include light-emitting diode arrays such as in backlightunits of displays. Displays are used to present visual information andstatus data and/or may be used to gather user input data. A displayincludes an array of display pixels. Each display pixel may include oneor more colored subpixels for displaying color images.

Each display pixel may include a layer of liquid crystals disposedbetween a pair of electrodes operable to control the orientation of theliquid crystals. Controlling the orientation of the liquid crystalscontrols the polarization of backlight from a backlight unit of thedisplay. This polarization control, in combination with polarizers onopposing sides of the liquid crystal layer, allows light passing intothe pixel to be manipulated to selectively block the light or allow thelight to pass through the pixel.

The backlight unit includes one or more light-emitting diodes (LEDs)such as one or more strings and/or arrays of light-emitting diodes thatgenerate the backlight for the display. In various configurations,strings of light-emitting diodes may be arranged along one or more edgesof a light guide plate that distributes backlight generated by thestrings to the LCD unit, or may be arranged to form a two-dimensionalarray of LEDs.

Although examples discussed herein describe LEDs included in displaybacklights, it should be appreciated that the LED control circuitry andmethods described herein can be applied to LEDs implemented in otherdevices or portions of a device (e.g., in a backlit keyboard or a flashdevice).

Mixed mode dimming of LEDs is sometimes performed for LEDs that receivea common supply voltage, by individually controlling the current throughone or more LEDs using multiple current control modes. Mixed-modedimming includes directly controlling the current through one or moreLEDs for LED currents above a knee point and controlling the currentbelow the knee point using pulse-width modulation (PWM) of the currentwith a fixed peak current. The fixed peak current is equal to the kneepoint current, which is also the minimum of the directly controlledcurrent.

For example mixed-mode dimming can be used for local dimming of displaybacklights to enhance the displayed content on the display (e.g., toenhance the brightness of bright regions of the displayed content byincreasing backlight brightness and to provide darker dark regions ofthe displayed content by reducing backlight brightness in thoseregions).

Mixed mode dimming can be provided by an LED driver that includes adirect current supply (e.g., a digital-to-analog converter that operatesa current-control transistor) and a PWM switch. However, providingmixed-mode dimming using a single DAC and a single PWM switch can havedisadvantages in terms of power loss, current accuracy, susceptibilityto noise, and driver area.

In accordance with various aspects of the subject disclosure, mixed modedimming of LEDs includes PWM dimming using multiple peak currents (e.g.,by varying a PWM duty cycle with each of two or more peak currents,modified using PWM on pulses). PWM dimming using multiple peak currentsmay help reduce headroom voltages at the end of one or more LED strings(e.g., the voltage at a location between a last LED in a series-coupledstring of LEDs and current control circuitry for that string), which canreduce power consumption by the device.

In accordance with various aspects of the subject disclosure, mixed modedimming of LEDs is provided using multiple drivers (e.g., a dual drivercircuit having a relatively smaller driver stage for PWM dimming and arelatively larger driver stage for providing direct current control). Adual driver circuit for LEDs as described herein can provide aselectable and/or trimmable direct-to-PWM transition current and/orother advantages as discussed in further detail hereinafter.

An illustrative electronic device having light-emitting diodes is shownin FIG. 1. In the example of FIG. 1, device 100 has been implementedusing a housing that is sufficiently small to be portable and carried bya user (e.g., device 100 of FIG. 1 may be a handheld electronic devicesuch as a tablet computer or a cellular telephone). As shown in FIG. 1,device 100 may include a display such as display 110 mounted on thefront of housing 106. Display 110 may be substantially filled withactive display pixels or may have an active portion and an inactiveportion. Display 110 may have openings (e.g., openings in the inactiveor active portions of display 110) such as an opening to accommodatebutton 104 and/or other openings such as an opening to accommodate aspeaker, a light source, or a camera.

Display 110 may be a touch screen that incorporates capacitive touchelectrodes or other touch sensor components or may be a display that isnot touch-sensitive. Display 110 may include display pixels formed fromlight-emitting diodes (LEDs), organic light-emitting diodes (OLEDs),plasma cells, electrophoretic display elements, electrowetting displayelements, liquid crystal display (LCD) components, or other suitabledisplay pixel structures. Arrangements in which display 110 is formedusing LCD pixels and LED backlights are sometimes described herein as anexample. This is, however, merely illustrative. In variousimplementations, any suitable type of display technology may be used informing display 110 if desired.

Housing 106, which may sometimes be referred to as a case, may be formedof plastic, glass, ceramics, fiber composites, metal (e.g., stainlesssteel, aluminum, etc.), other suitable materials, or a combination ofany two or more of these materials.

The configuration of electronic device 100 of FIG. 1 is merelyillustrative. In other implementations, electronic device 100 may be acomputer such as a computer that is integrated into a display such as acomputer monitor, a laptop computer, a somewhat smaller portable devicesuch as a wrist-watch device, a pendant device, or other wearable orminiature device, a media player, a gaming device, a navigation device,a computer monitor, a television, or other electronic equipment.

For example, in some implementations, housing 106 may be formed using aunibody configuration in which some or all of housing 106 is machined ormolded as a single structure or may be formed using multiple structures(e.g., an internal frame structure, one or more structures that formexterior housing surfaces, etc.). Although housing 106 of FIG. 1 isshown as a single structure, housing 106 may have multiple parts. Forexample, housing 106 may have upper portion and lower portion coupled tothe upper portion using a hinge that allows the upper portion to rotateabout a rotational axis relative to the lower portion. A keyboard suchas a QWERTY keyboard and a touch pad may be mounted in the lower housingportion, in some implementations. An LED backlight array may also beprovided for the keyboard and/or other illuminated portions of device100.

In some implementations, electronic device 100 may be provided in theform of a computer integrated into a computer monitor. Display 110 maybe mounted on a front surface of housing 106 and a stand may be providedto support housing (e.g., on a desktop).

FIG. 2 is a schematic diagram of display 110 in which the display isprovided with a liquid crystal display unit 204 and a backlight unit202. As shown in FIG. 2, backlight unit 202 generates backlight 208 andemits backlight 208 in the direction of liquid crystal display unit 204.Liquid crystal display unit 204 selectively allows some or all of thebacklight 208 to pass through the liquid crystal display pixels thereinto generate display light 210 visible to a user. Backlight unit 202includes one or more subsections 206.

In some implementations, subsections 206 may be elongated subsectionsthat extend horizontally or vertically across some or all of display 110(e.g., in an edge-lit configuration for backlight unit 202). In otherimplementations, subsections 206 may be square or other rectilinearsubsections (e.g., subarrays of a two-dimensional LED array backlight).Accordingly, subsections 206 may be defined by one or more stringsand/or arrays of LEDs disposed in that subsection. Subsections 206 maybe controlled individually for local dimming of backlight 208.

Although backlight unit 202 is shown implemented with a liquid crystaldisplay unit, it should be appreciated that a backlight unit such asbacklight unit 202 may be implemented in a backlit keyboard, or toilluminate a flash device or otherwise provide illumination for anelectronic device.

FIG. 3 shows a schematic diagram of exemplary LED circuitry such asbacklight circuitry for display 110. For example, LED circuitry 300 ofFIG. 3 may be implemented in backlight unit 202 or other LED lightingdevices. In the example of FIG. 3, circuitry 300 includes at least oneLED 302 (e.g., an LED in a string of series-coupled LEDs) and anassociated driver 301 for controlling the brightness of the LED.

In the example of FIG. 3, LED circuitry 300 includes switch 304,transistor 306, and resistor 308 coupled in series between LED 302 and aground voltage. Switch 304 is operated by PWM driver 301. Transistor 306is operated by controlling a gate voltage for the transistor with aselectable voltage reference 312 such as a digital-to-analog converter(DAC) coupled to the gate terminal. As shown in FIG. 3, an operationalamplifier 314 may be coupled between DAC 312 and the gate terminal oftransistor 306 to provide feedback control of the current throughtransistor 306. A first input terminal of amplifier 314 receives anoutput of DAC 312 and a second input terminal of amplifier 314 receivesa residual voltage for comparison, by amplifier 314 to the input voltagefrom DAC 312. The output of amplifier 314 includes an output terminalcoupled to the gate terminal of transistor 306. In the example of FIG.3, the feedback voltage is a residual voltage at a location betweentransistor 306 and resistor 308.

Driver 301 converts input brightness information to current levels todrive LEDs 302 (e.g., implemented in LED strings). LED current iscontrolled either using linear scaling of the current using DAC 312 tooperate current control transistor 306 or by PWM control using PWMdriver 310 at a fixed peak current to operate switch 304 such thataverage current through LED 302 is controlled by adjusting the dutycycle of a fixed frequency PWM waveform. FIG. 4 shows an example plot400 of LED current ILED vs. LED brightness in which an LED is controlledusing a PWM current 402 at low currents (e.g., currents below a kneepoint or switch (SW) point) and using linearly scaled current 404 atrelatively high current (e.g., at currents above the knee point). PWMbrightness control can help avoid color shifts at low LED currentlevels.

However, operating LED 302 using the currents illustrated in FIG. 4 witha single LED driver 301 as shown in FIG. 3 can have disadvantages, ifcare is not taken. For example, the headroom voltage at the end of oneor more LED strings (e.g., at a location between the last LED 302 in thestring and current control circuitry such as switch 304 and/ortransistor 306 for that string) can be high causing unwanted powerdissipation. As another example, in some scenarios, 12 bits of PWMresolution is desired which can lead to PWM duty cycles equal toapproximately 0.025% (e.g., with an approximately 10 ns pulse width fora PWM frequency of 25 kHz). This can lead to relatively narrow pulses toachieve high resolution, which can become distorted in time.

In accordance with some aspects of the subject disclosure, multi-peakPWM control of LED 302 can be performed, which can help reduce headroomvoltage and power dissipation. FIG. 5 shows an example plot 500 ofPWM-controlled LED currents 502 and 504 each having a different peakcurrent (e.g., respective peak currents of approximately 6.25 milliamps(mA) and 33 mA). As indicated in FIG. 5, the two peak currents may betunable to control the headroom voltage of the LED device.

FIG. 6 shows example headroom voltages for three strings 602 of LEDs 604of an LED device 600, operated with mixed mode dimming as shown in FIG.3. In each string 602, one or more LEDs 604 are coupled in seriesbetween a supply voltage Vout and a current controller 606 (e.g., driver301 of FIG. 3). Illustrative LED currents and resulting string voltages610 and headroom voltages 608 are shown. In the example of FIG. 6,direct controlled LED currents of 40 mA and 20 mA result in stringvoltages 610 of 6.1 Volts (V) and 5.7 V and headroom voltages of 0.7 Vand 1.1 V. A PWM current 612 of 7.5 mA results in string voltage of 5.4V and a residual voltage of 1.4 V.

As shown in FIG. 7, if the middle string 602 is instead operated with a20 mA PWM current 700 generated by a 40 mA peak current with a 50% PWMduty cycle, the headroom voltage on that string is reduced to 0.7 V. Asillustrated by FIGS. 6 and 7, operating LEDs using multi-peak PWMcurrents as in the example of FIG. 4, can facilitate the use of reducedheadroom voltages which can reduce the overall power consumption of anLED device.

The multi peak PWM control can be applied by changing the output of DAC312 of FIG. 3 to raise and/or lower the fixed peak current, or can beprovided by an LED driver circuit with multiple drivers. FIG. 8 shows anexample of exemplary LED circuitry such as backlight circuitry fordisplay 110 that includes multiple drivers. For example, LED circuitry800 of FIG. 8 may be implemented in backlight unit 202 or other LEDlighting devices. In the example of FIG. 8, circuitry 800 includes atleast one LED 802 (e.g., an LED in a string of series-coupled LEDs) andassociated drivers 801 and 821 (sometimes referred to as driver stagesof an overall driver for LED 802), coupled in parallel between LED 802and the ground voltage, for controlling the brightness of the LED.

In the example of FIG. 8, driver 801 includes switch 804, transistor806, and resistor 808 coupled in series between LED 802 and the groundvoltage. Switch 804 is operated by PWM driver 810. Transistor 806 isoperated by controlling a gate voltage for the transistor with adigital-to-analog converter (DAC) 812 coupled to the gate terminal. Asshown in FIG. 8, an operational amplifier 814 may be coupled between DAC812 and the gate terminal of transistor 806 to provide feedback controlof the current through transistor 806. A first input terminal ofamplifier 814 receives an output of DAC 812 and a second input terminalof amplifier 814 receives a residual voltage for comparison, byamplifier 814 to the input voltage from DAC 812. The output of amplifier814 includes an output terminal coupled to the gate terminal oftransistor 806.

In the example of FIG. 8, driver 831 includes switch 834, transistor836, and resistor 838 coupled in series between LED 802 and the groundvoltage. Switch 834 is operated by PWM driver 830. Transistor 836 isoperated by controlling a gate voltage for the transistor with adigital-to-analog converter (DAC) 832 coupled to the gate terminal. Asshown in FIG. 8, an operational amplifier 844 may be coupled between DAC832 and the gate terminal of transistor 836 to provide feedback controlof the current through transistor 836. A first input terminal ofamplifier 844 receives an output of DAC 832 and a second input terminalof amplifier 844 receives a residual voltage for comparison, byamplifier 844 to the input voltage from DAC 832. The output of amplifier844 includes an output terminal coupled to the gate terminal oftransistor 836.

Drivers 801 and/or 821 convert input brightness information to currentlevels to drive LEDs 802 (e.g., implemented in LED strings). When usingdriver 801 alone, LED current is controlled by linearly modifying thecurrent from DAC 812 or by setting a fixed peak current using DAC 812 tooperate current control transistor 806 and by reducing the averagecurrent through LED 802 by PWM control using PWM driver 810 to operateswitch 804. When using driver 821 alone, LED current is controlled bylinearly modifying the current from DAC 832, or by setting a fixed peakcurrent using DAC 832 to operate current control transistor 836 and byreducing the average current through LED 802 by PWM control using PWMdriver 830 to operate switch 834.

Driver 801 may be used when driver 821 is decoupled from LED 802 (e.g.,with switch 834). Driver 821 may be used when driver 801 is decoupledfrom LED 802 (e.g., with switch 804). For example, one of drivers 801and 821 may be a relatively larger than the other of drivers 801 and821. For very low brightness, only the small driver may be operationalto provide high resolution with lower distortion. In some scenarios,drivers 801 and 802 may both be operated to deliver LED current (e.g.,for high brightness operations). In some scenarios, driver 801 and 821may co-operate to provide multi-peak PWM control as described above inconnection with FIG. 5.

For example, for a first LED brightness, DAC 812 can provide a firstpeak current (e.g., an adjustable peak current of about 5-10 mA) whichcan be reduced, on average, by PWM controller 810 to a first averagecurrent corresponding to the first LED brightness. For a second LEDbrightness, DAC 832 can provide a second peak current (e.g., anadjustable peak current of about 30-40 mA) which can be reduced, onaverage, by PWM controller 830 to a second average current correspondingto the second LED brightness.

In other scenarios, high resolution PWM dimming of LED 802 can beperformed by inter-modulating the PWM control of drivers 801 and 821. Inother scenarios, mixed mode dimming of LED 802 can be performed by usingthe smaller one of drivers 801 and 821 for PWM dimming and the larger ofdrivers 801 and 821 for the linear current control. In accordance withsome aspects, mixed mode dimming of LEDs may be provided by a dualdriver LED control circuit in which only one of the drivers is PWMcontrollable.

FIG. 9 shows an example of LED circuitry such as backlight circuitry fordisplay 110 that includes multiple drivers in which only one of thedrivers is PWM controllable. In the example of FIG. 9, LED circuitry 900includes a first driver 921 and a second driver 901 coupled in parallelbetween LED 902 and a ground voltage.

Driver 921 may be operated to provide PWM controlled current through LED902 by operation of switch 944 by PWM controller 930 to selectivelycouple and decouple DAC 932 (e.g., an 8 bit DAC) from a first inputterminal of operational amplifier 934. As shown, a second input terminalof operational amplifier 934 receives a feedback voltage for comparisonto the PWM controlled voltage from DAC 932. The output terminal ofamplifier 934 is coupled to the gate terminal of transistor 936 forcontrolling current through LED 902. PWM controller 930 may providedithering (e.g., PWM controller may be a 14 bit controller that provides10 bits PWM resolution and 4 bits dithering). The peak value of thecurrent in the PWM cycle generated by driver 921 may be trimmed usingDAC 932. DAC 932 may provide, for example, a peak current of 5-10 mA(e.g., 6.25 mA plus 20 percent).

Driver 901 may be operated to provide linear current control for higherLED currents through LED 902 (e.g., direct currents of up to between 30mA and 40 mA, such as a 33.75 mA current). DAC 912 (e.g., a 10 bit DAC)selects the output current of linear driver 901. As shown, the output ofDAC 912 can be provided to a first input terminal of an operationalamplifier 914 that has an output terminal coupled to the gate terminalof transistor 904 (coupled in series between LED 902 and resistor 906).A second input terminal of amplifier 914 receives a feedback voltagefrom a location between transistor 904 and resistor 906.

In order to provide additional control of drivers 901 and 921, currentDACs (IDACs) such as IDACs 1000 and 1010 may be coupled, respectively toDACs 932 and 912 as shown in FIG. 10. In the configuration of FIG. 10,the transition point between PWM control by driver 921 and linearcontrol by driver 901 may be selectable using IDAC 1000 (e.g.,selectively coupling a current source 1002 to DAC 932 using switches1004). In the configuration of FIG. 10, trimming of the transition pointbetween PWM control by driver 921 and linear control by driver 901 maybe performed using trim values 1008 for DAC 932. In the configuration ofFIG. 10, analog control of the current through LED 902 can be performedusing only DAC 912. In the configuration of FIG. 10, display-widedimming of backlight unit 202 may be performed using IDAC 1010 (e.g., byselectively coupling a current source 1012 to DAC 912 using switches1014). In various implementations, IDACs 1000 and 1010 may be coupledone or more drivers of one or more LEDs provide local or globaltransition point selection and/or global dimming for a display.

LED circuitry as shown in FIG. 10 provides mixed mode current control,as shown by the current plot 400 of FIG. 11, in which PWM control isperformed at low currents by driver 921, linear current control isprovided at higher currents by driver 901, and the switch (SW) ortransition point between PWM control and linear control is bothtrimmable (e.g., by +20%/−44%) and scalable. LED circuitry as shown inFIG. 9 and FIG. 10, as examples, can provide reduced power loss, betteraccuracy, better noise immunity, and a smaller driver area (e.g., for a10 bit composite DAC/PWM driver relative to a 14 bit single DAC/PWMdriver).

FIG. 12 depicts a flow diagram of an example process for multi-peak PWMcontrol of LED current in accordance with various aspects of the subjecttechnology. For explanatory purposes, the example process of FIG. 12 isdescribed herein with reference to the components of FIGS. 3 and 8-10.Further for explanatory purposes, the blocks of the example process ofFIG. 12 are described herein as occurring in series, or linearly.However, multiple blocks of the example process of FIG. 12 may occur inparallel. In addition, the blocks of the example process of FIG. 12 neednot be performed in the order shown and/or one or more of the blocks ofthe example process of FIG. 12 need not be performed.

In the depicted example flow diagram, at block 1200, at least onelight-emitting diode is operated using a first pulse-width-modulationduty cycle and a first peak current. For example, the first PWM dutycycle and the first peak current may correspond to the duty cyclesand/or peak current of PWM current 502 of FIG. 5.

At block 1202, the at least one light-emitting diode is operated using asecond pulse-width-modulation duty cycle and a second peak current,where the first and second pulse-width-modulation duty cycles and/or thefirst and second peak currents are different. For example, the secondPWM duty cycle and the second peak current may correspond to the dutycycles and/or peak current of PWM current 504 of FIG. 5.

FIG. 13 depicts a flow diagram of an example process for multi-peak PWMcontrol of LED current in accordance with various aspects of the subjecttechnology. For explanatory purposes, the example process of FIG. 13 isdescribed herein with reference to the components of FIGS. 8-10. Furtherfor explanatory purposes, the blocks of the example process of FIG. 13are described herein as occurring in series, or linearly. However,multiple blocks of the example process of FIG. 13 may occur in parallel.In addition, the blocks of the example process of FIG. 13 need not beperformed in the order shown and/or one or more of the blocks of theexample process of FIG. 13 need not be performed.

In the depicted example flow diagram, at block 1300, at least onelight-emitting diode is operated using a first driver circuit (e.g.,driver 901 of FIG. 9 or FIG. 10) that provides a constant current to theat least one light-emitting diode.

At block 1302, the at least one light-emitting diode is operated using asecond driver circuit (e.g., driver 921 of FIG. 9 or FIG. 10) thatprovides a modulated (e.g., PWM) current to the at least one lightemitting diode.

In accordance with various aspects of the subject disclosure, anelectronic device having a display with a backlight is provided, thebacklight including a light-emitting diode and a backlight driver. Thebacklight driver includes a first driver stage coupled to thelight-emitting diode and having a pulse-width modulation controller anda first linear current controller. The backlight driver also includes asecond driver stage coupled to the light-emitting diode and having asecond linear current controller.

In accordance with other aspects of the subject disclosure, anelectronic device having a display is provided, the display including abacklight unit that includes a light-emitting diode, direct currentcontrol circuitry, and pulse-width-modulation current control circuitryto modulate a first current provided by the direct current controlcircuitry through the light-emitting diode and to modulate a secondcurrent provided by the direct current control circuitry through thelight-emitting diode. The first current is larger than the secondcurrent.

In accordance with other aspects of the subject disclosure, a method isprovided that includes operating at least one light-emitting diode usinga first pulse-width-modulation duty cycle and a first peak current andoperating the at least one light-emitting diode using a secondpulse-width-modulation duty cycle and a second peak current. The firstand second peak currents are different.

In accordance with other aspects of the subject disclosure, a method isprovided that includes operating at least one light-emitting diode usinga first driver circuit that provides a constant current to the at leastone light-emitting diode and operating the at least one light-emittingdiode using a second driver circuit that provides a modulated current tothe at least one light-emitting diode.

Various functions described above can be implemented in digitalelectronic circuitry, in computer software, firmware or hardware. Thetechniques can be implemented using one or more computer programproducts. Programmable processors and computers can be included in orpackaged as mobile devices. The processes and logic flows can beperformed by one or more programmable processors and by one or moreprogrammable logic circuitry. General and special purpose computingdevices and storage devices can be interconnected through communicationnetworks.

Some implementations include electronic components, such asmicroprocessors, storage and memory that store computer programinstructions in a machine-readable or computer-readable medium(alternatively referred to as computer-readable storage media,machine-readable media, or machine-readable storage media). Someexamples of such computer-readable media include RAM, ROM, read-onlycompact discs (CD-ROM), recordable compact discs (CD-R), rewritablecompact discs (CD-RW), read-only digital versatile discs (e.g., DVD-ROM,dual-layer DVD-ROM), a variety of recordable/rewritable DVDs (e.g.,DVD-RAM, DVD-RW, DVD+RW, etc.), flash memory (e.g., SD cards, mini-SDcards, micro-SD cards, etc.), magnetic and/or solid state hard drives,ultra density optical discs, any other optical or magnetic media, andfloppy disks. The computer-readable media can store a computer programthat is executable by at least one processing unit and includes sets ofinstructions for performing various operations. Examples of computerprograms or computer code include machine code, such as is produced by acompiler, and files including higher-level code that are executed by acomputer, an electronic component, or a microprocessor using aninterpreter.

While the above discussion primarily refers to microprocessor ormulti-core processors that execute software, some implementations areperformed by one or more integrated circuits, such as applicationspecific integrated circuits (ASICs) or field programmable gate arrays(FPGAs). In some implementations, such integrated circuits executeinstructions that are stored on the circuit itself.

As used in this specification and any claims of this application, theterms “computer”, “processor”, and “memory” all refer to electronic orother technological devices. These terms exclude people or groups ofpeople. For the purposes of the specification, the terms “display” or“displaying” means displaying on an electronic device. As used in thisspecification and any claims of this application, the terms “computerreadable medium” and “computer readable media” are entirely restrictedto tangible, physical objects that store information in a form that isreadable by a computer. These terms exclude any wireless signals, wireddownload signals, and any other ephemeral signals.

To provide for interaction with a user, implementations of the subjectmatter described in this specification can be implemented on a computerhaving a display device as described herein for displaying informationto the user and a keyboard and a pointing device, such as a mouse or atrackball, by which the user can provide input to the computer. Otherkinds of devices can be used to provide for interaction with a user aswell; for example, feedback provided to the user can be any form ofsensory feedback, such as visual feedback, auditory feedback, or tactilefeedback; and input from the user can be received in any form, includingacoustic, speech, or tactile input.

Many of the above-described features and applications are implemented assoftware processes that are specified as a set of instructions recordedon a computer readable storage medium (also referred to as computerreadable medium). When these instructions are executed by one or moreprocessing unit(s) (e.g., one or more processors, cores of processors,or other processing units), they cause the processing unit(s) to performthe actions indicated in the instructions. Examples of computer readablemedia include, but are not limited to, CD-ROMs, flash drives, RAM chips,hard drives, EPROMs, etc. The computer readable media does not includecarrier waves and electronic signals passing wirelessly or over wiredconnections.

In this specification, the term “software” is meant to include firmwareresiding in read-only memory or applications stored in magnetic storage,which can be read into memory for processing by a processor. Also, insome implementations, multiple software aspects of the subjectdisclosure can be implemented as sub-parts of a larger program whileremaining distinct software aspects of the subject disclosure. In someimplementations, multiple software aspects can also be implemented asseparate programs. Finally, any combination of separate programs thattogether implement a software aspect described here is within the scopeof the subject disclosure. In some implementations, the softwareprograms, when installed to operate on one or more electronic systems,define one or more specific machine implementations that execute andperform the operations of the software programs.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, declarative orprocedural languages, and it can be deployed in any form, including as astand alone program or as a module, component, subroutine, object, orother unit suitable for use in a computing environment. A computerprogram may, but need not, correspond to a file in a file system. Aprogram can be stored in a portion of a file that holds other programsor data (e.g., one or more scripts stored in a markup languagedocument), in a single file dedicated to the program in question, or inmultiple coordinated files (e.g., files that store one or more modules,sub programs, or portions of code). A computer program can be deployedto be executed on one computer or on multiple computers that are locatedat one site or distributed across multiple sites and interconnected by acommunication network.

It is understood that any specific order or hierarchy of blocks in theprocesses disclosed is an illustration of example approaches. Based upondesign preferences, it is understood that the specific order orhierarchy of blocks in the processes may be rearranged, or that allillustrated blocks be performed. Some of the blocks may be performedsimultaneously. For example, in certain circumstances, multitasking andparallel processing may be advantageous. Moreover, the separation ofvarious system components in the embodiments described above should notbe understood as requiring such separation in all embodiments, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

The previous description is provided to enable any person skilled in theart to practice the various aspects described herein. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects. Thus, the claims are not intended to be limited to theaspects shown herein, but are to be accorded the full scope consistentwith the language claims, wherein reference to an element in thesingular is not intended to mean “one and only one” unless specificallyso stated, but rather “one or more.” Unless specifically statedotherwise, the term “some” refers to one or more. Pronouns in themasculine (e.g., his) include the feminine and neuter gender (e.g., herand its) and vice versa. Headings and subheadings, if any, are used forconvenience only and do not limit the subject disclosure.

The predicate words “configured to”, “operable to”, and “programmed to”do not imply any particular tangible or intangible modification of asubject, but, rather, are intended to be used interchangeably. Forexample, a processor configured to monitor and control an operation or acomponent may also mean the processor being programmed to monitor andcontrol the operation or the processor being operable to monitor andcontrol the operation. Likewise, a processor configured to execute codecan be construed as a processor programmed to execute code or operableto execute code

A phrase such as an “aspect” does not imply that such aspect isessential to the subject technology or that such aspect applies to allconfigurations of the subject technology. A disclosure relating to anaspect may apply to all configurations, or one or more configurations. Aphrase such as an aspect may refer to one or more aspects and viceversa. A phrase such as a “configuration” does not imply that suchconfiguration is essential to the subject technology or that suchconfiguration applies to all configurations of the subject technology. Adisclosure relating to a configuration may apply to all configurations,or one or more configurations. A phrase such as a configuration mayrefer to one or more configurations and vice versa.

The word “example” is used herein to mean “serving as an example orillustration.” Any aspect or design described herein as “example” is notnecessarily to be construed as preferred or advantageous over otheraspects or design

All structural and functional equivalents to the elements of the variousaspects described throughout this disclosure that are known or latercome to be known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe claims. Moreover, nothing disclosed herein is intended to bededicated to the public regardless of whether such disclosure isexplicitly recited in the claims. No claim element is to be construedunder the provisions of 35 U.S.C. § 112, sixth paragraph, unless theelement is expressly recited using the phrase “means for” or, in thecase of a method claim, the element is recited using the phrase “stepfor.” Furthermore, to the extent that the term “include,” “have,” or thelike is used in the description or the claims, such term is intended tobe inclusive in a manner similar to the term “comprise” as “comprise” isinterpreted when employed as a transitional word in a claim.

What is claimed is:
 1. An electronic device having a display with abacklight, the backlight comprising: a light-emitting diode; and abacklight driver having: a first driver stage coupled to thelight-emitting diode and having: a pulse-width modulation controller;and a first linear current controller, wherein the first linear currentcontroller comprises: a first current control transistor having a firstsource/drain terminal coupled to the light-emitting diode, a secondsource/drain terminal coupled to a ground voltage via a resistor, and agate terminal; and a first digital-to-analog converter having an outputterminal coupled to the gate terminal of the first current controltransistor; and a second driver stage coupled to the light-emittingdiode and having a second linear current controller.
 2. The electronicdevice of claim 1, wherein the output terminal of the firstdigital-to-analog converter is coupled to the gate terminal of the firstcurrent control transistor, via a switch.
 3. The electronic device ofclaim 2, wherein the pulse-width modulation controller is configured tooperate the switch to provide pulse-width-modulation control of acurrent through the light-emitting diode and the first current controltransistor.
 4. The electronic device of claim 3, wherein the secondlinear current controller comprises: a second current control transistorhaving a first source/drain terminal coupled to the light-emittingdiode, a second source/drain terminal coupled to the ground voltage viaan additional resistor, and a gate terminal; and a seconddigital-to-analog converter having an output terminal coupled to thegate terminal of the second current control transistor.
 5. Theelectronic device of claim 4, wherein the first linear currentcontroller further comprise an amplifier coupled between the switch andthe gate terminal of the first current control transistor.
 6. Theelectronic device of claim 5, wherein the second linear currentcontroller further comprise an additional amplifier coupled between thesecond digital-to-analog converter and the gate terminal of the secondcurrent control transistor.
 7. The electronic device of claim 4, furthercomprising a first current digital-to-analog converter coupled to thefirst digital-to-analog converter.
 8. The electronic device of claim 7,further comprising a second current digital-to-analog converter coupledto the second digital-to-analog converter.
 9. An electronic devicehaving a display, the display comprising: a backlight unit, comprising:a light-emitting diode; direct current control circuitry, wherein thedirect current control circuitry comprises: a current control transistorcoupled in series between the light-emitting diode and a ground voltage;and a digital-to-analog converter coupled to a gate terminal of thecurrent control transistor; and pulse-width-modulation current controlcircuitry to modulate a first peak current provided by the directcurrent control circuitry through the light-emitting diode and tomodulate a second peak current provided by the direct current controlcircuitry through the light-emitting diode, wherein the first peakcurrent is larger than the second peak current.
 10. The electronicdevice of claim 9, wherein the pulse-width-modulation current controlcircuitry comprises a switch coupled between the light-emitting diodeand the current control transistor, the switch operable based on apulse-width-modulation signal from a pulse-width-modulation controller.11. The electronic device of claim 10, wherein the digital-to-analogconverter is operable to provide the first peak current and the secondpeak current using the current control transistor, and wherein, in somemodes of operation, the second peak current is unmodulated by thepulse-width-modulation current control circuitry.
 12. The electronicdevice of claim 10, wherein the digital-to-analog converter is operableto provide the first peak current using the current control transistorand wherein the direct current control circuitry further comprises anadditional digital-to-analog converter operable to provide the secondpeak current using an additional current control transistor.
 13. Theelectronic device of claim 12, wherein the pulse-width-modulationcurrent control circuitry further comprises an additional switch coupledbetween the light-emitting diode and the additional current controltransistor.
 14. A method of operating an electronic device, comprising:operating at least one light-emitting diode using a firstpulse-width-modulation duty cycle and a first peak current; andoperating the at least one light-emitting diode using a secondpulse-width-modulation duty cycle and a second peak current, wherein thefirst and second peak currents are different, wherein operating at leastone light-emitting diode using the first pulse-width-modulation dutycycle and the first peak current comprises providing the first peakcurrent using a digital-to-analog converter and providing the firstpulse-width-modulation duty cycle with a pulse-width-modulationcontroller.
 15. The method of claim 14, wherein the first peak currentis larger than the second peak current.
 16. The method of claim 14,wherein operating at least one light-emitting diode using the secondpulse-width-modulation duty cycle and the second peak current comprisesproviding the second peak current using the digital-to-analog converterand providing the second pulse-width-modulation duty cycle with thepulse-width-modulation controller.
 17. The method of claim 14, whereinoperating at least one light-emitting diode using the secondpulse-width-modulation duty cycle and the second peak current comprisesproviding the second peak current using an additional digital-to-analogconverter and providing the second pulse-width-modulation duty cyclewith an additional pulse-width-modulation controller.
 18. A method ofoperating an electronic device, comprising: operating at least onelight-emitting diode using a first driver circuit that provides aconstant current to the at least one light-emitting diode; and operatingthe at least one light-emitting diode using a second driver circuit thatprovides a modulated current to the at least one light-emitting diode,wherein the first driver circuit comprises a first digital-to-analogconverter coupled to a first current control transistor for the at leastone light-emitting diode, wherein the second driver circuit comprises asecond digital-to-analog converter coupled to a second current controltransistor for the at least one light-emitting diode, and wherein thefirst digital-to-analog converter is an N-bit digital-to-analogconverter, wherein the second digital-to-analog converter is an M-bitdigital-to-analog converter, and wherein N is greater than M.
 19. Themethod of claim 18, wherein operating the at least one light-emittingdiode using the second driver circuit that provides the modulatedcurrent to the at least one light-emitting diode comprises modulating acurrent from the second digital-to-analog converter using a switchcoupled between the second digital-to-analog converter and the secondcurrent control transistor.